I do have this hunch that the Captain still has some role to play which is a strange thing to say given that he's dead.
Same. I really don't think his "arc" is done, even though he may not actually be directly active in the story. The fact that this whole trilogy basically started with his character's intro also inclines me to believe there's more to the Captain's arc.
I go back and forth on him being a Ciphrang. I lean towards that not being the case, only because Bakker alludes to it so heavily that it feels like it must be a red-herring.
I have a crackpot that the Captain really isn't anything other than a man who had a "near-death experience". At some point he died and his soul witnessed true Hell, but he "bounced back" (to quote Sarl), either because he was somehow resuscitated or
maybe through magical means via Kellhus and/or Iyokus.
Regardless, having witnessed Hell, Kosoter was of course never the same man again. It was basically like looking into the Inverse Fire. He realized that no Worldly pain or horror could ever even come close to matching genuine Damnation, and so upon returning from death he was utterly changed. The atrocities he saw and committed became nothing to him, just like the Consult members that saw Damnation. Ironsoul seems a fitting name for the man in this case. This scenario also explains his comment in the lava-filled depths of Cil-Aujas when they all think they're in Hell, but Kosoter says that it isn't and he knows because
he'd already seen Hell. In addition, this explains his absolute, utter conviction to Kellhus. He knows Damnation is real in a way that few do, and for whatever reason he believes that Kellhus is his path to Salvation.